Components are sometimes coated with a material to locally modify the properties of the component. Coating the external surfaces of a component with a material is called surface coating. Surface coating of a component to improve surface properties saves money and time since desirable properties can be achieved without having to fabricate the component with a different material. For example, easy availability, low cost, and good pour qualities make cast iron a desirable material for cast engine components. However, cast iron components used in a corrosive environment may be susceptible to corrosion. The ability to coat the surfaces of the component with a corrosion resistant material may increase the corrosion resistance of the component without sacrificing the beneficial properties of cast iron. Surface coatings have been extensively used in industry to impart beneficial properties to components.
An especially difficult environment to provide protection for a metal substrate is one which combines a high temperature corrosive ambient with wear, as seen in turbocharger housings and exhaust components of internal combustion engines. Surface coatings of metal carbide or metal nitride, such as titanium carbide (TiC), titanium nitride (TiN), or chrome nitride (CrN, Cr2N), are sometimes used to provide abrasion and corrosion resistance for components in these extreme environments. Typically, these coatings are applied by ion coating processes, such as physical vapor deposition (PVD), chemical vapor deposition (CVD), or by means of a galvanic coating process. It has been observed that the CrN, Cr2N, TiN or TiC coatings have a tendency to peel off over time and may need to be reapplied. Although these metal nitrides and metal carbide coatings may be reapplied, it may be advantageous to use a more durable coating material that would last a longer time. Additionally, in the case of some components, the reapplication processes of these coatings may be cumbersome.
Another type of surface coating used in industry to increase corrosion and wear resistance of metal components are conversion coatings. Conversion coatings are surface coatings where the part of the metal surface is converted into the coating with a chemical or electrochemical process. Examples include chromate conversion coatings, and phosphate conversion coatings. Phosphate conversion coating treatments (called “phosphating”) provide a coating of insoluble metal-phosphate crystals that adhere strongly to the base metal. Typically, these phosphating treatments are applied to a metal surface before painting. Generally, phosphating solutions are prepared from liquid concentrations containing one or more divalent metals, free phosphoric acid, and an accelerator. The phosphating process consists of a series of application and rinse stages typically involving the application of either an iron, or zinc phosphate solution to a substrate. A simple iron phosphating system is composed of two stages: an iron phosphate bath that both cleans the part and applies the conversion coating followed by a rinse bath to remove dissolved salts from the treated surface. Following the conversion application, the components may be dried.
Although the application process of conversion coatings may be suitable for reapplication, the wear and corrosion protection offered by these coatings may not be significant. Therefore, a surface coating for a metal substrate that provides good corrosion and wear resistance that can be applied using a process suitable for reapplication may be desired.